Accumulation of damaged cellular proteins is postulated to be a major contributor to aging and sensitivity to cell damage. Decreases in both protein synthesis and degradation rates may result in the persistence of defective or modified proteins and thus the overall rate of protein turnover can affect the cells' response to cell damage.
Elongation factor-2 kinase (eEF2 kinase) is a ubiquitous protein kinase that belongs to the alpha-kinase family of protein kinases. these protein kinases do not display sequence homology to conventional eukaryotic protein kinases (Ryazanov, A. G. et al. Proc. Natl. Acad. Sci. USA 94, 4884-4889 (1997); Ryazanov, A. G., Pavur, K. S., Dorovkov, M. V. Curr. Biol. 28, 943-945 (1999); and Ryazanov, A. G. FEBS Lett. 514, 26-29 (2002)). The only known substrate of eEF2 kinase is eEF2, the protein that promotes ribosomal translocation during the elongation phase of protein synthesis. eEF2 is inactivated upon phosphorylation by eEF2 kinase, implying that the phosphorylation process catalyzed by eEF2 kinase can be a mechanism of protein synthesis regulation at the elongation stage (Ryazanov, A. G., et al. Nature 334, 170-173 (1988)). eEF2 kinase is a highly specific protein kinase, which phosphorylates and inactivates elongation factor-2 (Ryazanov, A. G., et al. Nature 324, 170-173 (1988); and Ryazanov, A. G. et al. Proc. Natl. Acad. Sci. U.S.A. 94, 4884-4889 (1997)). This kinase is regulated by the IGF-1/mTOR pathway and is implicated in the control of the global rate of protein synthesis at the elongation stage (Browne, G. J., Proud C. G. Eur. J. Biochem. 269, 5360-5368 (2002); and Proud C. G. Curr. Top. Microbiol. Immunol. 279, 215-244 (2004)). eEF2 kinase activity in cells is normally low, but is stimulated by various stress-inducing agents such as Ca2+ ionophores, acidic pH and hydrogen peroxide (Nairn, A. C. and Palfrey, H. C. Cold Spring Harbor Laboratory Press, 295-318 (1996); Patel, J. et al., Eur. J. Biochem. 269, 3076-3085 (2002); and Dorovkov, M. V., et al. Biochemistry 41, 13444-13450 (2002)).
To uncover the physiological role of eEF2K eEF2K knockout mice are prepared. Despite a complete lack of eEF2K activity, eEF2K knockout mice have normal development, behavior and reproduction. Moreover, these mice have increased lifespan. However, fibroblasts from eEF2K knockout mice are found to be resistant to various cytotoxic agents. The effect of eEF2K knockout on cell resistance to cytotoxic agents may depend on functional p53 since it is abolished in cells in which p53 is inactivated. Intriguingly, knockout mice have significantly extended maximal lifespan. These findings suggest that eEF2K is a modulator of stress resistance and aging, and that its inactivation could protect cells from stress-induced injury and increase life span in mammals.
One of the hallmarks of aging is the progressive decline in the rate of protein synthesis and degradation. This decline in protein turnover can be a major factor contributing to an increase in the concentration of damaged proteins with age. Therefore, by regulating the overall rate of protein synthesis and/or degradation it might be possible to modulate the rate of aging. Since knockout of eEF2 kinase increases maximal life span in mice, manipulation of expression or activity if eEF2 kinase may offer a therapeutic basis for regulating protein turnover and reducing cell damage due to stress, exposure to chemotherapeutic agents, and other factors.